Pre-flight Background Estimates for COSI

TL;DR

This work provides pre-flight estimates of COSI's MeV gamma-ray background by combining updated cosmic-ray and albedo spectra with GALPROP Galactic diffuse emission models, simulated in MEGAlib/Cosima. It accounts for time-dependent geomagnetic effects, SAA passages, and delayed activation from short- and long-lived isotopes, using a memory-based activation technique over the first 3 months of orbit and extrapolating to 2 years with a decay-based scaling factor. The results show extragalactic background dominates at energies below ~660 keV, while delayed activation from CR primaries and albedo photons dominates at higher energies; activation after SAA passages and long-term build-up of isotopes significantly shape the background, including prominent lines near the 511 keV and near the 1332 keV region. The methodology, including a comparison to AP9/AE9 via HEPD-01 data and a detailed long-term isotope extrapolation, provides a solid foundation for optimizing COSI's observing strategy, calibration plans, and background mitigation throughout its mission. The work emphasizes the modularity of the background model, enabling iterative improvements with commissioning data and future instrument calibrations.

Abstract

The Compton Spectrometer and Imager (COSI) is a Compton telescope designed to survey the 0.2 - 5 MeV sky, consisting of a compact array of cross-strip germanium detectors. It is planned to be launched in 2027 into an equatorial low-Earth (530 km) orbit with a prime mission duration of 2 years. The observation of MeV gamma rays is dominated by background, mostly from extragalactic and atmospheric photon but also from the activation of the detector materials induced by cosmic-ray interactions. Thus, background simulation and identification are crucial for the data analysis. In this work we perform Monte Carlo simulations of the background for the first 3 months in orbit, and we extrapolate the results to 2 years in orbit, in order to determine the build-up of the activation due to long-lived isotopes. We determine the rates of events induced by the background that are reconstructed as Compton events in the simulated COSI data. We find that the extragalactic background photons dominate at low energies (<660 keV), while delayed activation from cosmic-ray primaries (proton/alpha) and albedo photons dominate at higher energies. As part of this work, a comparison at low latitude (<1 deg) between recent measurement of the SAA by the High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite (CSES-01) and the AP9/AE9 model has been made, showing an overestimation of the flux by a factor 9 by the model. The systematic uncertainties associated with these components are quantified. This work marks a major step forward in estimating and understanding the expected background rates for the COSI satellite mission.

Figures (16)

• Figure 1: Full on-orbit BG spectra for charged particles, neutrons, and photons. All spectral components are divided by the solid angle of their region of origin. The average rigidity cutoff (12.6 GV) for COSI’s orbit has been used to generate the spectra (see section \ref{['sec:GC']}).
• Figure 2: All-sky map of the SA100-F98 model for the Galactic diffuse emission background component.
• Figure 3: Left: Image of the MEGAlib's mass-model used for the simulation. Right: CAD drawing (version 2025) of COSI payload for comparison.
• Figure 4: Left: Schematic showing the 22° rocking occurring every 12 hours. right: Pointing of the instrument's z-axis as a function of Galactic latitude and longitude for the 3 months of orbit.
• Figure 5: $R_{\mathrm{cutoff}}$ distributions as a function of time for a few hours of COSI orbit.